A passive fiber optic star is a device used to distribute the optical information from one fiber optic source to several fiber optic receivers simultaneously, without an external source of power. The heart of a fiber optic star is its mixing element, a device by which the optical signal coming in from any one of several input fibers is distributed more or less evenly among the output fibers. Characteristic features of mixing elements include the number of input and output ports, connectorization method, uniformity, insertion loss and excess loss. Insertion loss is the amount of attenuation experienced between an input and output port. Excess loss is the amount of attenuation of the input signal before reaching the output ports.
It is already known to employ a slab type mixer as a fiber optic star. For example, the U.S. Pat. No. 4,213,670 issued to Milton et al on July 22, 1980 discloses a planar fiber optic star wherein the fibers from the transmission line cable are assembled side-by-side along the width of a flat, rectangular in cross section, ribbon fiber mixing element to form a planar optical coupler. The input and output optical fibers are in line so that they are coupled to opposite ends of the planar mixer. The ribbon has a width greater than the width of all the transmission fibers and has a thickness less than that of the core diameter of the fibers. Shims having a thickness substantially the same as the fiber cladding thickness are used to align the fiber with the ribbon.
U.S. Pat. No. 4,362,357 issued to Stockmann et al on Dec. 7, 1982 shows a planar star coupler having input and output fibers arranged in line, like Millon et al, with the fibers extending from opposite sides of the coupler. Stockmann et al also has an embodiment with a rigid semicircular glass mixer which allows the input and output fibers to extend from the same side of the device. The two groups of fibers lie in the same plane and they are spread over a width at least as great as the maximum dimension of the semicircular mixer.
U.S. Pat. No. 4,484,794 issued to Witte on Nov, 27, 1984 and U.S. Pat. No. 4,449,783 to Witte on May 22, 1984 each disclose a mixer element formed by a planar waveguide having a thickness which is approximately equal to the diameter of the fiber elements. These also require the input fibers to be in line and extending from opposite sides of the device.